With a new leather harness, oil the leather two to three times before use. Oil soaks into the leather and makes it soft. You can compare leather to muscle fibres running parallel to each other. And when you put oil on it, those fibres glide along each other more easily. Be careful not to oil too much. Then the fibres glide too easily again and you pull the whole leather apart, so to speak.
Greasing leather
After oiling, you put the leather in grease. Grease does not soak in, but stays on and thus protects the leather. After using the harness, clean the leather with lukewarm water or a little soapy water. Do not use too much and do not scrub. For regular maintenance, use grease every now and then. Oiling should be done no more than once or twice a year. If you use the harness a lot, it will also stay soft and then you don't even need to oil it much. Compare it to your shoes. You put grease on those now and then because it gets bald, but not because it dries out. Daily use keeps leather supple.
Plastic harness maintenance
Plastic has the advantage that you virtually don't need to maintain it. You can clean it with an ordinary water jet. Do not use high pressure because you will destroy the stitching. If the plastic becomes dull or stiff, use a little green soap or silicone spray. This will make the synthetic material nice and smooth again.
Juliette Post/The Menner
